In recent years, there has been known a technique by which a mobile station UE performs “discontinuous reception” of downlink data, in order to save its own battery, in a radio access scheme such as “LTE (long term evolution)”, the 3GPP standardization of which is underway.
In this radio access scheme, it is not always necessary to maintain uplink synchronization between the mobile station UE in a discontinuous reception state and a radio base station eNB.
Accordingly, this radio access scheme is configured to establish uplink synchronization between the radio base station eNB and the mobile station UE, as shown in FIG. 1, when downlink user data for the mobile station UE in the discontinuous reception state occurs and when uplink synchronization is not maintained between the mobile station and the radio base station eNB.
To be more precise, as shown in FIG. 1, in Step S3001, the radio base station eNB transmits “UL synch request (an uplink synchronization request)” to the mobile station UE, in order to establish uplink synchronization between the radio base station eNB and the mobile station UE.
In Step S3002, using an asynchronous RACH (random access channel), the mobile station UE transmits a dedicated preamble designated by the above-mentioned “UL synch request”.
Here, the dedicated preamble means a data series which is individually allocated to each mobile station. Note that the “UL synch request” does not include the dedicated preamble itself but includes an identifier for identifying the dedicated preamble.
In Step S3003, in response to the dedicated preamble transmitted from the mobile station UE, the radio base station eNB creates a TA (Timing Advance) command for adjusting uplink transmission timing for the mobile station UE and transmits the TA command to the mobile station UE.
However, the mobile station UE fails to receive the TA command and therefore retransmits the above-described dedicated preamble by using the asynchronous RACH in Step S3004.
In Step S3005, the radio base station eNB creates a TA command in response to the dedicated preamble retransmitted from the mobile station UE and transmits the TA command to the mobile station UE.
Thereafter, in Step S3006, the radio base station eNB determines that uplink synchronization is established since having received no dedicated preamble from the mobile station UE for a predetermined period. Then, the radio base station eNB transmits the above-mentioned downlink user data to the mobile station UE by using a DL-SCH (downlink shared data channel: Downlink-Shared Channel).    Non-patent Document 1: 3GPP TSG RAN WG2 #57, R2-070781